c++ - Why using different (shared_ptr and normal) pointer constructors on the same class I get different results? -

inspired this question wrote own little program (using code this answer) , discovered things confused me. namely:

having classes:

class {   public:     a(){}     int i;     int j; };  class b {   public:     b() = default;     int i;     int j; }; 

i wrote 4 usages of classes different object construction usages:

//1 a* pa = new a(); // followed delete pa b* pb = new b(); // followed delete pb //2 shared_ptr<a> pa = make_shared<a>(); shared_ptr<b> pb = make_shared<b>(); //3 shared_ptr<a> pa ( new a() ); shared_ptr<b> pb ( new b() ); //4 shared_ptr<a> pa ( new ); shared_ptr<b> pb ( new b ); 

and after have used them print values so:

for( int = 0; < 3; ++i) {   ...1..2..3..4   cout << "a : " << pa->i << "," << pa->j << endl;   cout << "b : " << pb->i << "," << pb->j << endl; } 

what have received on output surprised me:

//1 : 0,0 b : 0,0 : 7388176,0 b : 0,0 : 7388208,0 b : 0,0  //2 : 0,0 b : 0,0 : 0,0 b : 0,0 : 0,0 b : 0,0  //3 : 0,0 b : 0,0 : 25848848,0 b : 0,0 : 25848880,0 b : 0,0  //4 : 0,0 b : 0,0 : 39619600,0 b : 39619664,0 : 39619632,0 b : 39619696,0 

while first 1 described in aforementioned question's answers happening in //2, //3, //4?

i used gcc 4.9.2 , 5.1.0 on ubuntu 64 bit same results.

ideone link

your first 3 code snippets value-initialize objects constructed.

if class's default constructor not user-provided, value-initialization performs zero-initialization, , call default constructor; otherwise, calls default constructor.

now apply discussion in question linked.

for make_shared case see zeroes in both cases, that's 1 possible manifestation of undefined behavior. nothing more.

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the final snippet default-initializes objects constructed, which, object of class type, means calling default constructor, in each case performs no initialization.